The present invention relates to a method of correcting a taper in a grinding machine and a taper correcting apparatus for the same.
Conventionally, a cylindrical grinding machine is provided with a parallelism correcting apparatus in order to correct parallelism between a center line connecting a tail stock defining the center of a workpiece and a head stock defining the center of a workpiece and a center line of a grinding wheel spindle.
For example, U.S. Pat. No. 3,690,072 discloses a structure in which grinding wheel bearings are supported by an eccentric housing, and a grinding wheel spindle can be tilted by rotating the housing.
Unexamined Japanese Patent Application Publication No. Hei. 3-281156 discloses a structure in which an elastic flexible portion is disposed in a grinding wheel spindle housing which holds grinding wheel bearings, and the grinding wheel housing can be tilted about the elastic flexible portion by a toggle mechanism.
Unexamined Japanese Patent Application Publication No. Hei. 7-195266 discloses a structure in which a table on which a head stock and a tail stock are mounted can be displaced in a horizontal plane.
Generally, in a grinding machine which grinds a workpiece having an untrue circular shape, such as a crank shaft or a cam shaft, the workpiece is rotated while one end of the workpiece is supported by a tail stock on a workpiece spindle stock and the other end is clamped by a workpiece chuck of a workpiece spindle. The rotation axis of the workpiece is made parallel with the rotation axis of plural grinding wheels disposed on a wheel spindle stock, and the grinding wheels are moved back and forth in a grinding direction or in a direction perpendicular to the rotation axis of the workpiece. An inclination of the rotation axis of the grinding wheels with respect to the rotation axis of the workpiece is adjusted on the basis of data of outer diameters of ground surfaces of end portions of the workpiece which are measured by plural sizing devices during a work of grinding the workpiece, thereby reducing a taper of the outer circumferential face of the workpiece to "zero," i.e., making the outer diameters of ground surfaces of end portions of the workpiece equal to each other.
In a process of designing such a grinding machine, the rotation axis of a workpiece rotated while being supported by the workpiece supporting mechanism is set to be parallel with the rotation axis of the grinding wheels supported by the wheel spindle stock, and the plural grinding wheels are set to have the same diameter. When a workpiece is ground, however, the outer diameters of ground surfaces of end portions of the workpiece are often different from each other because of production errors of components of the grinding machine and assembly errors, with the result that the ground surface of the workpiece is tapered.
In an initial stage of operation where a work grinding work by a grinding machine is started, particularly, the temperature of the grinding machine is gradually raised, and the temperature change causes components constituting the grinding machine to thermally expand in different manners. As a result, even when workpieces are ground under the same conditions, a difference between outer diameters of ground surfaces of end portions of each workpiece, i.e., the taper accuracy of each workpiece is largely changed. In an initial stage of operation, for example, first to fifth workpiece grinding works, the inclination of the rotation axis of the grinding wheels with respect to the rotation axis of the workpiece which is required for correcting a taper of a workpiece is largely changed. Conventionally, the taper correction is not performed at all during a period from a timing when the grinding wheels are moved from their standby position to that immediately before the grinding wheels are contacted with the ground surface of the workpiece. After the work of grinding the workpiece by the grinding wheels is started, the taper correction is performed by feedback controlling a servomotor of a taper correcting apparatus on the basis of data of outer diameters of end portions of the workpiece which are measured by sizing devices.
In a grinding machine, plural rest devices are disposed at positions which are opposed to the grinding wheels, respectively, in order to prevent the workpiece from being pressed during a grinding work by the grinding wheels at a degree in excess of that needed. Following to a change of the grinding amount, advance positions of the rest devices which are contacted with the workpiece, or the pressing forces of the rest devices exerted on the ground surface of the workpiece are controlled.
In the above-described taper correction, a method of correcting a taper of a workpiece in an initial stage of operation of a grinding machine is performed after a grinding wheel is contacted with a ground surface of the workpiece. Therefore, the method has a problem in that the time period required for the taper correction is long and hence the time period of grinding a workpiece cannot be shortened. The reason of the above will be described.
Referring to FIG. 22 which schematically shows an apparatus for correcting a taper of a workpiece, an inclination of a rotation axis O2 of grinding wheels 91, 92, and 93 with respect to a rotation axis O1 of a workpiece W and about a fulcrum O is adjusted by a motor M1. In the adjustment of the inclination, the state where the rotation axes O1 and O2 are parallel with each other is set as a reference position, and the rotation axis is moved in the same direction as that in which the grinding wheels 91, 92, and 93 are advanced by a motor M2 and which is perpendicular to the rotation axis O1 of the workpiece W. When the direction of the taper correction is identical with the grinding direction, a correction point P1 of the rotation axis O2 of the grinding wheels 91, 92, and 93 is subjected to a correction to the minus side in which the cutting amount of the workpiece W is increased and the reduction of the outer diameter of the right-end pin portion W3 of the workpiece W is larger than that of the outer diameter of the left-end pin portion W1. When the directions are opposite to each other, a correction to the plus side is performed in which the reduction of the outer diameter of the right-end pin portion W3 of the workpiece is smaller than that of the outer diameter of the left-end pin portion W1. In the case where only the taper correction to the plus side is to be performed, even when the taper correction is performed during a grinding work, it is not required to reduce the advancing speed of the grinding wheels by the motor M2. Therefore, the time period of grinding is not prolonged. When the workpiece is ground while performing the taper correction to the plus side, however, phenomena such as increase of distortion of the workpiece and impairment of the surface roughness occur. As a result, actually, both the advancing speed of the grinding wheels and that of the taper correction are reduced. By contrast, the correction to the minus side must be performed during a grinding work while the operation of advancing the grinding wheels is stopped. In this case, therefore, there arises a problem in that the time period of a grinding work is further prolonged as the taper correction amount is larger.
These problems are conspicuous in an initial stage of operation of a grinding machine, but occur also in the steady operation. A steady operation region where the grinding work by a grinding machine has been performed a predetermined number of times and mechanisms of the grinding machine stably operate is a stable region where the amount of each taper correction in works of the same kind is reduced and the amount of the taper correction in each of works is not substantially changed. However, the taper correction is performed during a grinding work, and hence the time period of the grinding work is prolonged by the correction.
In consideration of the above, a configuration in which the taper corrections to the plus and minus sides on a correction point of a workpiece is not entirely performed or, even when such a correction is performed, the correction amount is very small is ideal for shortening the time period of a grinding work, reducing distortion of a ground surface of a workpiece, and improving the accuracy such as distortion of a ground surface of the workpiece and the surface roughness.
On the other hand, the rest devices are advanced and retracted by a servomotor, and perform only positioning. Alternatively, rapid advancement and retraction are performed by means of positioning, and other operations are controlled by means of a torque. Thereafter, the apparatus for correcting a taper of a workpiece is operated so as to correct a difference between outer diameters of ground surfaces of end portions of a workpiece, i.e., a taper. During this correction, conventionally, the advance positions of the rest devices which delicately affect the difference between outer diameters, and the pressing force on the ground surface are not corrected. In a grinding machine in which the taper correction amount of a workpiece is largely changed, therefore, a failure in the advance position of a rest device or in a pressing force causes the function of the workpiece for supporting the ground surface, to be excessively increased or deficiently reduced, thereby producing a problem in that the accuracies of the workpiece such as the roundness and the straightness are unstable.